The present invention relates to an evaporative emission control system for an internal combustion engine, and more particularly to such a system that the emission of evaporative fuel is prevented by maintaining the pressure in a fuel tank at a negative pressure.
For example, Japanese Patent Laid-open No. 11-50919 discloses an evaporative emission control system including a passage for connecting a fuel tank directly to an intake system of an internal combustion engine (this passage will be hereinafter referred to as "first evaporative fuel passage") to maintain the pressure in the fuel tank at a negative pressure (a pressure lower than the atmospheric pressure). This conventional system further includes a canister for temporarily storing evaporative fuel, and a passage for connecting the fuel tank through the canister to the intake system of the internal combustion engine (this passage will be hereinafter referred to as "second evaporative fuel passage"). With this configuration, tank purge for purging the evaporative fuel from the fuel tank through the first evaporative fuel passage to the intake system and canister purge for purging the evaporative fuel stored in the canister through the second evaporative fuel passage to the intake system can be carried out in parallel.
In the above conventional system, however, the flow of the evaporative fuel in the tank purge and the canister purge is set to such an amount that having almost no effects on air-fuel ratio control for the engine. Accordingly, in the case that the pressure in the fuel tank is equal to the atmospheric pressure or in the case that the canister is in a full-charged condition, much time is required for each purge.
Further, in the above conventional system, a canister purge amount, i.e., an evaporative fuel amount to be supplied to the intake system by the canister purge, is first determined. Then, the canister purge amount is subtracted from a maximum evaporative fuel amount that can be purged to determine a tank purge amount, i.e., an evaporative fuel amount to be supplied to the intake system by the tank purge. As a result, the tank purge amount is relatively small, causing a delay of pressure reduction in the fuel tank. Conversely, in the case of adopting a method of preferentially determining the tank purge amount, there occurs another problem that the canister purge amount becomes lacking to cause a reduction in storage capacity of the canister (especially, the storage capacity required in refueling).